This invention relates generally to the water purification field. In particular, it is concerned with a system for recovering treated water used to backwash and rinse filters used to remove manganese and iron impurities from water.
It is known to add treatment chemicals to water containing iron and manganese ions and to filter the treated water to remove such ions. U.S. Pat. No. 3,167,506 to Fackler et al describes one such method in which permanganate ions are employed in conjunction with a manganese oxide zeolite filter bed. Other treatment chemicals, such as chlorine and pH adjustment chemicals like sodium hydroxide, typically are also added to the water being treated.
As the water passes through the manganese oxide zeolite bed, the iron and manganese ions dissolved therein are oxidized into an insoluble oxide and/or hydroxide either by the zeolite material operating as an oxidizing agent (in which event the filter must be periodically regenerated with a strong oxidizing agent, such as for example potassium permanganate), or by the catalytic effect of the manganese oxide zeolite bed. These oxidation mechanisms for precipitating iron and manganese ions are more fully described in the above-identified Fackler patent.
After the filtering system has been in operation for a period of time, the filter bed becomes partially clogged by the precipitates. It is thus desirable to "back wash" the filter periodically by passing treated water in the direction opposite to normal filtering flow, i.e. from the filter outlet through the filter to the filter inlet, to remove the precipitates from the filter and restore the mechanical and catalytical efficiency of the bed. In processes heretofore employed, the treated backwash water containing suspended matter has been disposed of by merely depositing it in a sewer or natural body of water.
During backwashing, the filter granules tend to become dislodged or unpacked so that the filter bed does not properly treat the first water passed through it after backwashing. It is conventional to "rinse" the filter with treated water immediately after backwashing. In rinsing, new water is passed through the filter in the direction of normal filtering flow to repack the filter granules. Heretofore, the rinse water also has been disposed of merely by depositing it in a sewer or the like.
Inasmuch as the cost of treatment chemicals used to treat the backwash and rinse water frequently amounts to on the order of 3 to 12% of the cost of operating the filtering system, such disposal of backwash water results in significant waste and lack of economy in the overall operation of the system. A concomitant disadvantage of the prior method of disposing of the backwash and rinse water is that by placing the impurities containing backwash water into sewers or the like, the water treatment system emits wastes which contribute to pollution of other water sources, besides wasting the washwater itself.
It is, therefore, desirable to provide a system for recovering the treated water used to backwash and rinse filter beds of this type.